The advent of the Winchester disk storage devices has substantially increased the capacity and speed with which digital information can be stored and retrieved by microprocessor based systems.
As demands for increased storage capacity, reduced size, and high access speed have grown, manufacturers have made various improvements in Winchester disk drives to meet those demands. For example, to increase storage capacity, manufacturers have increased the number of disks stacked on the spindle, increased the usable disk surface on a disk of a given diameter, and have increased the bit density in each disk track. They have also increased the number of tracks per inch on the disk to increase storage density. As track densities increase, however, the dimensional stability of the disk-spindle assembly and acceptable tolerances for radial disk movement become critical, and high mechanical precision is required.
In prior art disk drives the disks are mounted over a spindle such that the internal diameter of each disk fits around the external diameter of the spindle with a typical locational clearance fit of 0.0035/0.0005 inches. Thus, the disks, which are intended to be stationary on the spindle, fit securely thereon but can be readily assembled and disassembled from the spindle.
Unfortunately, the dimensional relationship between the outer diameter of the spindle and the inner diameter of the disks in prior art Winchester disk drives does not remain constant. The spindle is typically ferrous so that it will carry the magnetic flux of the integral motor field magnets. The disks are typically aluminum. Temperature changes that occur within the specified operating and storage temperature range of the disk drive often causes radial movement of the disks with respect to the spindle due to differential thermal expansion of their different materials.
As a result, excessive errors are introduced during operation of the drive due to track eccentricity. Write operations must be performed with the read/write head directly on the centerline of the track. When the disks are displaced radially from their thermally stabilized position, however, a track written at the stabilizing temperature condition will be shifted off-center if it is later read under a different temperature condition. This is particularly critical with respect to the servo disk which is essential to ensuring accurate positioning of the heads.
There are two conditions under which the above-described disk eccentricity off-tracking problem most frequently arises. The first condition is where the drive is exposed to a temperature extreme outside its operating range, such as may be encountered during shipping of the drive in an airplane cargo bay. A typical operating temperature range for a Winchester disk drive, such as of the drive type known as the Maxtor XT-1000.TM. is 50.degree. C. to 4.degree. C. In a low temperature environment such as that described above, the drive may be exposed to temperatures of -40 degrees c'. Subsequently, when the drive is operated at or near room temperature, excessive errors will result due to permanent radial disk offset suffered during differential thermal expansion of the disks during temperature transients.
The second frequent error condition typically occurs when low or high end operating range temperatures are approached. Winchester disk drives, which are typically assembled at room temperature and are usually dimensionally stable at that temperature. If one or more disks are abutting the spindle at room temperature, and the drive is exposed to temperatures at the low end of its desired operating range, an elastic radial movement will cause a temporary degradation in tracking performance until the drive is brought back to its original temperature condition.
The problem of disk eccentricity due to thermal gradient-induced radial disk movement becomes even more acute as track densities increase. For example, where tracks are 4 mils wide, 400 micro-inches of radial movement will not result in excessive off-tracking errors, since it represents a movement equal to only 10% of track width. However, where the tracks are 650 micro-inches wide, 400 micro-inches of movement, which is 80% of the track width, will result in excessive tracking error.